U3 L1: Overview of Humoral Responses and Antibody Structure

What are the invariant chains that make up the BCR

Igα (Cd79a) and Igβ (Cd79b)

What kind of motif do Igα (Cd79a) and Igβ (Cd79b) in their cytoplasmic tails, and what does this allow them to do?

immunoreceptor tyrosine based activation motif (ITAM) that allows them to signal when the B cell receptor binds to antigen

(basic) what are the types of chains that make up the BCR and how many of each are present?

two identical light chains (kappa or lambda), and two identical heavy chains

For which chains of the BCR does VDJ recombination occur?

for both the heavy and the light chains

Which terminal on the heavy and light chains of the BCR are the antigen binding sites?

N terminal

What region in the BCR allows the Ig to be flexible

hinge region in each heavy chain

What holds the separate protein chains together in a BCR

disulfide bonds

CDRs (acronym meaning)

complementatity-determining regions

CDR definition

3 hypervariable regions in both the leavy chain and light chain regions; protruding loops that create the antigen binding surface

What two enzymes have been vital to understanding Ig structure

Papain and Pepsin

where does Papain cleave an Ig molecules

It cleaves at the top of each hinge region (above the disulfide bonds) leaving the Fc region and two separated Fab fragments

Where does pepsin cleave an antibody?

the bottom of each hinge region (below the disulfide bond), leaving an Fc portion and two joined fragments with antigen-binding activity called F(ab’)2 fragments

What happens after B cells get activated

they undergo clonal expansion (rapid proliferation)

Following clonal proliferation of an activated B cell, what do most of the resulting cells become?

antibody-producing plasma cells

Following clonal proliferation of an activated B cell, what do a minority of the resulting cells become?

memory B cells

What is the function of memory B cells

respond rapidly to cognate antigen in the future

What factors help stimulate activated B cells to proliferate?

helper T cells and other stimuli

What are three changes in antibody structure that can occur during humoral responses

1. affinity maturation

2. change from membrane to secreted form

3. isotype switching

does isotype switching impact affinity for antibody?

no

When an antibody changes from membrane to secreted form, is there a change in antigen recognition affinity

no

Do Ig’s secreted by plasma cells have a transmembrane region

there is no transmembrane region and there may also be changes to the heavy chain

fun fact: how many antibody molecules to plasma cells produce/second?

2-10k

What are the two possible fates for plasma cells after the infection has cleared?

1. short-lived plasma cells die off as part of clonal contraction

2. long-lived plasma cells take up residency in the bone marrow

Do memory B cells every become plasma cells?

no

If memory B cells never become plasma cells directly, how to they contribute to the adaptive immune response upon secondary exposure to the antigen?

they persist for long periods of time and can reignite clonal expansion after a second antigen exposure

What is the secondary antibody response dependent on?

CD4 cell help

What is the antibody isotype during the primary response?

Usually IgM > IgG

What is the antibody isotype during the secondary response?

Relative increases in IgG and, under certain situtations, in IgA or IgE

Antibody affinity during the primary response

lower average affinity, more variable

antibody affinity in the secondary response

higher average affinity (affinity maturation)

what is the primary response induced by?

all immunogens

What is the secondary response induced by?

only protein antigens (because only TD responses leaad to memory)

What feature of an antibody determines the angles/orientation that are possible for binidng multiple antigens?

the amount of flexibility from the Ab Hinger regions

Describe the state of the antibody-antigen complexes in the zone of antigen excess

small complexes

Describe the state of the antibody-antigen complexes in the zone of equivalence

large complexes

Describe the state of the antibody-antigen complexes in the zone of antibody excess

small complexes

List the 4 effector functions of antibodies:

1. neutralize microbes and toxins

2. opsonize microbes for phagocytosis

3. sensitize infected cells for antibody-dependent cellular cytotoxicity

4. activate the complement system

How do antibodies influence some microbes’ ability to penetrate epithelial barriers?

Antibodies can block microbial entry through the epithelial barrier

How do antibodies function to neutralize toxins?

antibodies block the binding of toxins to cells and thus inhibit the pathological effects of the toxins

How does antibody-mediated opsonization work?

microbe is obsonized by IgG → Fc receptors bind to the Ab coating the microbe → down stream signal in cell to activate phagocytosis → phagocycosis → internal killing of microbe

How does ADCC work

Antibodies bind to a foreign surface antigen expressed on a cell → NK cell with Fc receptor binds these antigens → NK cell kills the antibody coated cell

How to antibodies contribute to the complement system?

• IgM: the soluble planar form of the Ab binds to the bacterial surface and transitions to the stable form → One C1q binds the bound IgM

• IgG: IgG (monomers) bind bact. surface → C1q binds to at least two IgG molecules

• Both: binding of C1q leads to activation of the C3 convertase and cleavage of C3 → this leads to MAC, opsonization, and increased inflammation

Class switching - what is it?

AKA isotype switching; changes the portions of the constant regions in the Ig’s heavy chain; does not alter the variable region

When does class switching occur?

in general, after B cell activation

Principle effector functions of IgM

complement activation (remember, going from planar to stable form and recruiting C1q)

principle effector functions of IgG1-IgG3

opsonization and phagocytosisl complement activation; neonatal immunity via placental transfer

Primary functions of IgE and IgG4

immunity against helminths, and mast cell degranulation (immediate hypersensitivity)

principal effector functions of IgA

mucosal immunity (transport of IgA through epithelia)

Which antibody isotypes present as multimers?

IgM (pentamer) and IgA (dimer in some cases via the joining chain and secretory protein)

Where is secreted IgM primarily found

blood and some inflamed tissue

What does IgM excel at?

agglutination and fixing C1q (acivating complement system)

Where are IgGs found?

throughout the body → the ab with the most widespread distribution

stability of IgG

very stable, long half-life

What is the difference between the different IgG subclasses

• they have different hinge regions

• different affinities for C1q (IgG3, IgG1, IgG2, IgG4)

• different affinities for Fc receptors

What is the major class of antibodies that facilitates protection at mucosal surfaces?

IgA

Where is IgA found in its monomeric form?

blood

Where is IgA found in its dimeric form

mucosal secretions (mucous, tears, saliva, sweat, colostrum, genitourinary tract secretions, GI, prostate, respiratory epithelium)

composition of dimeric IgA

two IgAs joined by one joining chain and a secretory component that winds around the constant regions

primary function of IgA

neutralization

how is dimeric IgA transported into the gut lumen

through epithelial cells at the base of crypts (there is a poly Ig receptor on the basolateral side of the epithelial cells that helps the IgA get through

How is IgE unique among antibody isotypes

it is the only antibody that will bind to its Fc (FcER) receptor without first binding antigen

What are the main cells that express FcER receptors?

Mast cells, eosinophils, and basophils

What was IgE evolved to defend against?

multicellular parasites

where is IgE found

skin and connective tissues

what happens once IgE binds directly to FcER on mast cells

the FcER coplex now acts like aa “receptor” and when the Ab comes into contact with microbe, it crosslinks and causes the mast cell to degranulate

Fc receptors

cell surface glycoproteins with affinity for Fc regions

Which antibody transports across the epithelium?

IgA

Which antibody sensitizes mast cells?

IgE

Which antibodies transport across the placenta

IgG1, and IgG3 mainly

Which antibodies diffuse into extravascular sites?

all IgGs and monomeric IgA